Bicycles are typically provided with a crank shaft assembly that propels the bicycle when operated by a rider. Crank assemblies typically comprise a crank shaft that extends through a bottom bracket formed into the lower portion of a bicycle frame and pedal arms that are coupled to the crank shaft.
The limitation of known systems is that, while they may effectively be used with multi-piece crank shaft assemblies, they cannot effectively be used with standard-size single-piece crank shaft assemblies. For example, rather than have a crank shaft assembly comprise a crank shaft and two detachable pedal arms, it may be desired based upon the application to provide an integrated assembly. However, a single-piece structure would comprise a pedal arm extending in a first direction that transitions into the crank shaft extending at essentially a right angle from the pedal shaft, which then transitions into the other pedal arm that extends again at a right angle from the crank shaft and in an opposite direction from the first pedal arm. The result is that the unitary structure cannot readily be inserted through the relatively close fitting standard-size bottom bracket. This is not a problem for multi-piece crank shaft assemblies as the crank shaft may be inserted into the bottom bracket and then the pedal arms attached. While it is possible to manufacture an oversized bottom bracket assembly, this is undesirable as this greatly increases the weight of the assembly.
A number of systems have attempted to provide additional versatility for the bottom bracket assembly with limited success. For example, U.S. Pat. Nos. 4,252,384; 846,239; 648,657; and 641,907 all disclose a bottom bracket assembly that is variable in size. The bottom bracket assembly is provided with an opening in the housing and two tab portions extending on either side of the opening such that when the screw or bolt is tightened, it draws the opening closed. While this does provide some versatility for insertion and removal of the crank shaft assembly, a major drawback for these systems is fatigue of the bottom bracket assembly. This is especially problematic for frequent changing/adjustment of the crank shaft assembly. The problem is fundamental to the design because the bottom bracket assembly necessarily must flex open and close when changing or adjusting the crank shaft assembly leading to relatively early failure of the bottom bracket. Additionally, there is also a constant residual stress induced in the bottom bracket, reducing the ability of the assembly to withstand extreme outside forces. In other words, the entire assembly becomes substantially weaker if the part is pre-stressed.
Another system that has attempted to provide additional versatility is U.S. Pat. No. 547,990 entitled “Frame Connection For Bicycles.” This system employs a two-piece removable assembly that couples to the three bicycle supporting tubes that converge at the bottom bracket. As can be seen in the figures, the top piece fits in and around all three of the tubes, while the bottom piece fits part way around two of the tubes and attaches to the top piece. While this assembly provides additional versatility, the strength of the device is seriously compromised, which is especially critical in high impact applications, such as mountain biking and off-road applications. The reference teaches that a brazing technique may be used, however, this requires increased labor and cost and still does not provide adequate strength. Another problem with this arrangement is that if the tubes are provided as a composite material, the brazing technique cannot be used. This would result in a bicycle that is only held together by a couple of screws.
Still another system is disclosed in U.S. Pat. No. 5,209,581, which is directed to a crank arm mounting apparatus for a bicycle. This reference states that when the crank arms are “integrated with the crank shaft, a split bottom bracket is used which is hinged at one side thereof” and that after “placing the crank shaft unit in this bracket, the other free sides of the brackets are rigidly interconnected by bolts or the like.” (Col. 4, In. 67-Col. 5, In. 4.) While this system does provide for increased versatility, this system does not provide adequate adjustment and sealing of the bottom bracket assembly. For example, the reference teaches that the bottom bracket may be loosened on one side and opened when the opposite side is attached via a hinge arrangement. The system necessarily will be looser or open on the hinge side to allow the bottom portion to effectively rotate open and closed. This system only allows for tightening of one side of the bottom bracket therefore, when the hinged side loosens over time, there is no way to tighten it. For mountain biking and off-road applications, this will allow an unacceptable amount of dirt and/or debris into the bottom bracket resulting in damage and decreased performance. In addition, a hinge will, over time, be subject to corrosion, thereby making opening and closing difficult and negatively affecting the strength of the bracket assembly. Also, optimum bearing pre-load is difficult to obtain consistently, and so as to allow for free rotation of the bearings, the hinged side must be minimally tightened, thereby allowing the possibility of unexpected loosening of the bolts due to variable loading and vibration and also unexpected movement of the crank shaft assembly, which is highly undesirable.